The present invention relates to a method of melting treatment of radioactive miscellaneous solid wastes containing waste components of various kinds of substances such as metals, glass and the like, wherein the wastes are induction-heated effectively in a cold crucible induction melting furnace so that the volume of the wastes is reduced and the wastes are solidified.
Nuclear facilities and the like generate large amounts of radioactive miscellaneous solid wastes and a variety of kinds of substances such as combustibles, metals, glass and other non-combustibles are contained in the wastes. As a method of melting treatment of the wastes collectively in a simple manner, there is the one that was proposed by the applicant of the present application. (see Japanese Patent Laid-open No. 7-63895/1995, hereinafter referred to as the xe2x80x9cprior art methodxe2x80x9d).
The above-mentioned prior art method makes use of a cold crucible induction melting furnace having a structure in which a slit-divided water-cooling type metallic cold crucible is disposed within a water-cooling type high-frequency coil. When radioactive miscellaneous solid wastes are charged into the melting furnace and a high-frequency current is supplied to the high-frequency coil, electrically conductive substances such as metals contained in the miscellaneous solid wastes are melted first by being induction-heated. The remaining portion of the miscellaneous wastes surrounding the electrically conductive substances and having a low electrical conductivity is indirectly heated by the heat generated with the induction heating. That is, the metals act as a starting source of heating and melting the wastes so that the wastes reach a molten state entirely.
By the above-described prior art method, when the metals are melted, since a floating force works on the molten metal itself by the action of an electromagnetic field, the molten metal does not come into direct contact with the crucible. Also when the glass is melted, the contact surface of the molten glass with the crucible is cooled to become a solid layer (skull layer) and the high-temperature molten glass does not come into direct contact with the crucible. Thus, no high-temperature erosion of the crucible takes place. Further, since the crucible itself is water-cooled, it can melt the molten matter at a high temperature without being limited to its heat-resistance temperature.
However, in the above-described prior art method, a metallic substance, for example, contained in the miscellaneous solid wastes is first heated and melted by the application of an electrical current having a single frequency suitable for induction-heating the metallic substance to a single high-frequency coil, and glass and combustibles contained in the wastes are then indirectly heated by using the heat generated in melting the metallic substance as a starting heat source. Therefore, it takes much time to uniformly melt the whole of the wastes so that it is difficult. for the furnace to exhibit its optimum melting performance corresponding to the quality of the solid wastes to be subjected to a melting treatment.
An object of the present invention is therefore to provide a method of melting treatment of radioactive miscellaneous solid wastes wherein, even when the radioactive miscellaneous wastes contain various kinds of substances in a mixed state, the wastes can be induction-heated in the most suitable manner for each of the substances and can be entirely melted efficiently and quickly.
According to the method of melting treatment of radioactive miscellaneous solid wastes of the present invention, the radioactive miscellaneous solid wastes containing waste components of various kinds of substances are charged into a water-cooling type cold crucible induction melting furnace disposed within a high-frequency coil and a high-frequency electrical current is supplied to the high-frequency coil surrounding the melting furnace to melt the solid wastes by being induction-heated. The above-described procedure is the same as the prior art method. The present method is characterized in that a plurality of high-frequency currents each having a frequency suitable for induction-heating each of the various kinds of substances contained in the radioactive miscellaneous solid wastes are supplied to the high-frequency coil.
In the present invention described above, even when the miscellaneous solid wastes contain waste components of various kinds of substances in mixed state, since a plurality of high-frequency electrical currents of frequencies suitable for induction-heating various kinds of substances depending upon the electrical conductivity of the respective substances can be applied to the high-frequency coil, the whole of the wastes can be melted quickly and efficiently.
When a plurality of high-frequency currents are supplied to the high-frequency coil, these high-frequency currents may have different frequencies, and may be simultaneously supplied to a single high-frequency coil, or a plurality of high-frequency coils may be arranged around the melting furnace and the above-mentioned plurality of high-frequency currents may be supplied to the high-frequency coils, respectively.